1. Field of the Invention
The present invention relates in general to a marine propulsion system, and more particularly to a seal between components of the propulsion system of an outboard drive.
2. Description of Related Art
Many types of marine outboard drives discharge engine exhaust beneath the water level of the body of water in which the outboard drive is operated in order to silence exhaust noise. To facilitate such submerged emission of exhaust gases, exhaust systems of outboard drives commonly discharge exhaust gases to a low pressure region in the water produced by the propulsion device of the outboard drive. Discharge at this location produces an exhaust flow through the exhaust system without the presence of an undesirably high back pressure within the exhaust system. Propulsion systems conventionally include an exhaust discharge passage with an outlet positioned behind a propulsion device of the system for this purpose.
Many propulsion systems include joints or seams between one or more rotating components of the propulsion system. These joints often extend into the exhaust discharge passage. For instance, a propulsion system which includes a pair of counter-rotational propellers commonly includes an exhaust discharge passage through the center of each propeller boss. A joint usually exists between the two propellers, and extends from the exterior of the propellers into the exhaust discharge passage. As such, the exhaust discharge passage is not sealed throughout the propulsion system.
Operation of the propulsion device commonly produces elevated water pressure on the exterior side of the device. Because this exterior water pressure significantly exceeds the pressure within the exhaust discharge passage under some operating conditions, water tends to invade the exhaust discharge passage through any open joint or seam between the components of the propulsion system. The invasive water occupies space within the discharge passage and reduces the flow area through which exhaust gases can be discharged. As a result, the pressure within the exhaust system increases which consequently limits the output power produced by the engine of the outboard drive.
Under other operating conditions, the exhaust gas pressure is greater than the water pressure, and exhaust gases are admitted through the joint between the propellers. Emission of exhaust gas in this location can cause cavitations in the water about the propellers and reduce the efficiency of the propulsion system.